In recent years there have been many new developments in signal processing, coding and spread spectrum techniques that facilitate receivers' ability to separate and correctly decode multiple packets transmitted simultaneously over a channel. A better physical (PHY) layer alone will improve network performance, but the overall improvement could be far greater if there is suitable cooperation with the multiple access control (MAC) layer when scheduling packet transmissions. This idea is known as cross-layer design for networks with multipacket reception (MPR) capabilities.
Cross-layer design with MPR capabilities has not been applied previously to the practical design and theoretical analysis of carrier sense multiple access (CSMA) communications and all of its variants, such as the collision-detection variant (CSMA/CD). These systems assume that their underlying PHY layers offer a reception model which considers that a transmitted packet can be successfully decoded only if it is received in the absence of any other transmitted packet. Such systems employ channel-accessing protocols that refrain from transmitting whenever the channel is sensed busy and resume if it becomes idle again. These are the principles behind the popular Ethernet (IEEE 802.3) standard for local area networks (LANs). In these systems, the channel is the common wire medium—usually a coaxial cable or in recent years perhaps a fiber optic line—connecting each station in the network. By sensing the signal level on the wire, stations become aware of when another station already has started transmission.
Throughout the 1980s researchers began to analyze MAC performance for PHY layers using MPR methods, including spread spectrum techniques and showed that such PHY layers typically outperform those of the collision channel model. However, these ideas have not been applied. For wired systems, this is evident in the IEEE 802.3 standard for different transmission mediums, in the literature and in the patent literature. Spread spectrum techniques have been used in some fiber optic networks, but they have not been used in conjunction with other network communication techniques.
In the widely adopted IEEE 802.11 wireless LAN (WLAN) standards, first ratified in 1999, CSMA was chosen as the MAC layer's access method. In order to combat narrowband interference in the unlicensed radio-bands where these standards and other RF applications co-exist, spread spectrum or orthogonal frequency division multiplexing (OFDM) techniques are employed in the PHY layer. The 802.11 standards have opted to use these techniques for mitigating interference among nearby WLANs whose chosen channel bandwidths partially or wholly overlap. All of these standards, and therefore the equipment built to be compliant with them, still treat the underlying PHY layer as a collision channel. This has also been the case to date for non-standards-compliant CSMA-based networks.